The protest resulted in the brief detention of 99 individuals, all of them issued citations by the RCMP for trespassing after climbing over police barricades near the foot of the Peace Tower.

The immediate focus of the demonstration was the proposed expansion of Kinder Morgan's Trans Mountain pipeline from Alberta to Burnaby, B.C., which the Liberals have said they'll decide upon by mid-December.

But the larger theme was keeping fossil fuels in the ground, as many signs proclaimed, and urging Trudeau to keep his word on Canada's international emissions-cutting promises.

On Monday, the World Meteorological Organization released its 2015 inventory of greenhouse gases in the atmosphere and found that, on average, there were 400 parts per million of carbon dioxide in earth's atmosphere. That compares to about 278 parts per million before the industrial revolution.

The report predicts that "2016 will be the first year in which CO2 at the Mauna Loa Observatory remains above 400 ppm all year, and hence for many generations."

It is that cumulative increase that pipeline protesters insist doesn't allow for more expansion of fossil fuels such as Alberta's oilsands.

"Climate Leaders Don't Build Pipelines,'' said a giant banner carried at the front of the protest group, which was dominated by university students from Toronto, Montreal and Ottawa.

Forcing arrest was the point: student

Protest organizers called it the largest act of student climate civil disobedience in Canadian history, but the boisterous rally was a polite affair.

After some initial pushing and shoving at the police barricades, the protesters began individually climbing over the gates, often with police assistance, where they were then charged. The first dozen or so were handcuffed before being led away, but most of the detained protesters were not.

Andrew Stein, a McGill University environmental sciences student, said forcing the police to arrest them was the point of the exercise.

"It gets attention and it gets the word out there that climate leaders do not build pipelines,'' Stein said in an interview shortly before climbing the barricade himself.

"If Trudeau wants us on his team in 2019, he cannot approve this (Trans Mountain) pipeline." —Amanda Harvey-Sanchez, protest spokeswoman

Protest spokeswoman Amanda Harvey-Sanchez, a third-year University of Toronto student, said pipeline approvals are a deal-breaker for many younger voters who helped propel the Trudeau Liberals to a majority government in last October's general election.

"If Trudeau wants us on his team in 2019, he cannot approve this (Trans Mountain) pipeline,'' said Harvey-Sanchez.

"We're coming here to the capital to call on Trudeau to reject Kinder Morgan.''

Protest organizers said the 99 detained individuals, including Stein and Harvey-Sanchez, were issued citations that bar them from Parliament Hill for three months, but they were not fined.

Natural Resources Minister Jim Carr. (Photo: CP)

Natural Resources Minister Jim Carr shrugged off the protest, saying "dissent is the hallmark of democracy.''

"We've been saying all along that environmental stewardship and economic growth go hand-in-hand in Canada,'' he said.

"We have already announced — and we will continue to announce — very aggressive measures to reduce greenhouse gas emissions, always mindful of job opportunities for Canadians in the clean technology sector and in the energy sector overall.''